1. Field of the Invention
This invention relates to a wall transformer unit. More particularly this invention relates to a wall transformer unit for supplying regulated voltage to a handle connected thereto by an electrical cable. The handle attaches to an instrument that requires closely regulated power.
2. Description of the Prior Art
Wall transformer units for supplying power to hand-held instruments are required to supply highly regulated voltages in certain applications, such as medical instrumentation. Wall transformer units are typically calibrated at the factory. After a period of use, however, changes in resistance which occur as a result of aging, or as a result of the use of replacement parts, can cause variations in the output voltage that appears across the load at the handle.
Conventional wall transformer designs place an on/off switch in series with a power rheostat and in series with the load. Often a series microswitch is included in the circuit to activate the unit when the handle is disengaged from its cradle. The precision of the power rheostat is often poor, particularly at the minimal resistance extreme of its range, and at times a trimmer resistor is included elsewhere in the circuit to compensate. Nevertheless, replacement of components such as the coiled cord that extends from the wall unit to the handle, and aging effects are sufficient to produce an unacceptable variation in the output voltage. Recalibrating the trimmer is then necessary, but is inconvenient, and often not feasible in the field.
One known prior art device has attempted to deal with the problem of output voltage variation by an arrangement that employs a rheostat that is not in series with the load. However this device has the drawback of employing a carbon element in the rheostat that is imprecise in the region of minimal resistance. Furthermore the unit employs a series microswitch that has a non-zero resistance, which adversely affects calibration of the unit.
It is required in wall transformer units that the output turn on automatically when the handle is removed from its cradle on the unit. In past implementations, there was a fundamental problem in achieving this with a microswitch, because mechanical switches require more actuation force than that created by the weight of the handle. The prior art offers two typical solutions. In one approach the actuation force is artificially reduced by adding additional length to the microswitch actuating arm. This produces more leverage for the handle to work against, but decreases reliability, increases component variability and increases cost due to the extra parts. In another approach, a locking feature is added in the cradle, and the handle is held against the switch actuating arm with additional force. This also allows the product to function, but seriously decreases the ease of use. Instead of merely lifting the handle, the user must "pry" it out of its lock. In medical applications where there is high frequency of use and time is critical (e.g. hospital emergency rooms, clinics, etc.) the lock becomes an impediment, and is therefore a major disadvantage.